The present invention relates to biological wastewater treatment systems, and more particularly, to a bioreactor system for multi-stage biological wastewater treatment based on spatial microorganisms successions and trophic hydrobionts chains.
It is known that a spatially segregated trophic microorganism chain provides conditions at which larger organisms consume smaller ones. Such a spatial microorganism succession forms a basis for purification processes by means of both aerobic and anaerobic destruction of microorganisms. The result of such a succession is an increased efficiency of biochemical treatment and a reduced quantity of surplus biomass. Such purification processes are suitable for both domestic and industrial wastewaters, even those containing high levels of organic and inorganic impurities. Characteristically, systems for wastewater treatment by trophic microorganism chains include bioreactors having modular spatial aerobic and anaerobic units. Each bioreactor is provided with a controlled air supply that maintains the oxygen level needed for the activity of the microorganisms and enhances the biomass exchange.
Prior art bioreactors suffer from various drawbacks. Trickling filters require a large space, generate secondary pollution including bad odors, and attract flies. Activated sludge processes generate large amounts of biomass that need careful monitoring due to sudden changes in biomass loading and plugging. Rotational bioreactors are more compact, however, they are expensive and prone to mechanical problems.
It is known that bioreactors using fixed submerged biomass usually perform well at low biomass loadings, but are easily plugged by excessive buildup of biomass, therefore, demanding periodic cleaning or replacing of the submerged biomass. These prior art fixed submerged biomass bioreactors require many bioreactors to keep the loading low to maintain the purification efficiency. Consequently these wastewater-treating facilities demand frequent monitoring, good control of flow and load, and are expensive to install, operate and maintain.
U.S. Pat. No. 4,005,010 to Lunt describes a wastewater treatment system having mesh sacks containing a biological medium. The sacks are apparently designed to hold the microbes while allowing fluids to pass through. The biological medium is prone to clogging over the course of operation.
U.S. Pat. No. 4,165,281 to Kuriyama, et al., describes a wastewater treatment system that includes a substrate designed to contain the microorganisms. A plurality of vertically disposed substrates is designed for wastewater to pass therethrough. The likelihood of plugging is greater in this unit than in the Lunt device, due to the orientation of the substrates and to the difficulty in maintaining and/or replacing them.
U.S. Pat. No. 4,279,753 to Nielson, et al., describes the arrangement of a plurality of treatment reactors alternating from aerobic to anaerobic action. While Nielson indicates that it is necessary to address plugging problems, the technique for doing so is relatively crude and appears to be less than completely effective.
U.S. Pat. No. 4,521,311 to Fuchs, et al., teaches the use of a filtering bed through which the wastewater passes and which includes support bedding to suspend the biological medium. The device has a rather complex recirculation process in order to ensure cleaning of the bedding and the microbes. This device may experience additional kinds of clogging problems, and the disclosed bedding particles are required to go through a costly maintenance operation.
U.S. Pat. No. 5,221,470 to McKinney describes a wastewater treatment plant having a final filter made of a sheet of plastic. The sheet of plastic is wrapped about itself so as to form passageways designed for microbe growth. While this design may increase the surface area and, therefore, the dwell time available for microbial action, it is likely that plugging will occur as the passageway fills with dead microbes over a period of time.
In summary, prior-art bioreactor systems for multi-stage biological wastewater treatment are often plagued by inefficiency over a period of operation. When the wastewater to be treated requires the use of a considerable amount of biological mass, there results a problem of plugging of the mass. As waste solids build up on the surface of the mass, or as microbes ingest the pollutants and die, such solids do not always fall to the bottom of the bioreaction tank. Instead, the solids become trapped at or near the surface of the mass. This plugging or blocking of the mass significantly reduces the pathways by which subsequent pollutants may pass through to underlying active microbes that are located below the surface of the mass. Consequently, the acceleration of pollutant decay caused by microbe ingestion is compromised, and water flow through the mass is reduced and may even be stopped. It is therefore necessary to either build a substantially larger bioreactor unit than would otherwise be required—in order to account for this plugging—or to regularly clean the clogged system.
There is therefore a recognized need for, and it would be highly advantageous to have a bioreactor system for multistage biological wastewater treatment that is robust and efficient, simple to operate, insusceptible to plugging, and inexpensive to install and maintain.